Unfortunately, you need to do what others have suggested. It's no fun. You seem like you want to just be a good person about it. Sometimes, being a good person means getting pretty darn nasty.
Also, a welfare check is potentially putting police in unnecessary danger. If this dude is legitimately aware that he is in violation of laws concerning firearms, he might go off instead of face jail. It's best to at least give the cops the courtesy of knowing this is more than someone who might have broken a hip in their apt.
I know it's not stateside law enforcement, but in 2005 we had a tip in Iraq that seemed like a standard meet n greet of the locals. The Ministry of the Interior Commandos failed to tell us they had intel that it was a freakin' AQI training camp. We rolled into the situation thinking we were going to interview simple fishermen and started catching RPGs and machine gun fire. Had we known, it would have been a whole different approach. DON'T. DO. THAT. to the cops. Give them proper intel and let THEM decide if they want to roll in with nothing more than a smile.
